Spirent TestCenter: Doing an Autonegotiation settings mismatch between 2 back to back ports does NOT bring the port down.

Doc ID    KNI16146
Version:    1.0
Status:    Published
Published date:    06/22/2020


When using 1000Base-SX (optics/fiber - 1G speed) and do an Autonegotiation mismatch (Setting one port with AN=ON and other port with AN=OFF) and hit APPLY, the link remains up, however, setting AN=OFF in both ports, hit APPLY and then change one of the ports to AN=ON, and HIT apply again, now it brings the link in one side down.

The link should be down when an autonegotation mismatch between 2 ports.


  • FX2-1G-S16
  • Optics / Fiber
  • 1000Base-SX
  • SFP Ports


This beahvior is expected, after you acquire link with AN enabled (AN=ON), both sides are already transmitting and receiving Idle. We don’t issue any resets when you disable AN (AN=OFF) so link will remain up because both ports are still transmitting and receiving Idle. Then when you enable AN (AN=ON) on one port, link will be down because now this port is transmitting AN codewords and it is not receiving AN from the other port. 

Root Cause

  • WAD (Working as design)

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